How do you specify the size of a planet?
Diameter of planets are measured in kilometers.
Astronomers can know the distance to planet by radar measurements, then they measure the angular diameter. By using trigonometry we can calculate the diameter.
The largest planet in our solar system is Jupiter. It has a diameter of
There are a number of ways of specifying the size of a planet.
Planets in the solar system are often given a size which is their radius in kilometres.
The radius isn't sufficient to give a good comparison. Mass is another important metric. The masses of planets can be specified in kilograms. They can also be specified in terms of the number of Earth masses. Larger exoplanets may have their mass specified as the number of Jupiter masses.
For large objects the unit of mass is the solar mass. Our sun being 1 solar mass.
Using the solar mass scale Jupiter is about 0.001 solar masses. The largest a planet can be is about 13 Jupiter masses or 0.013 solar masses. Anything larger than 0.013 solar masses is large enough to start fusion reactions and is a brown dwarf or if more massive a star.